As is known, some circuit applications, such as audio amplifier applications, prefer or require active devices that are electrical complements of each other. For example, in audio amplifier applications that use vertical double-diffused metal oxide semiconductor field effect transistors (DMOS FETs), P-channel MOSFETs are coupled to their electrical complements, N-channel MOSFETs, to create a composite vertical DMOS device that provides a desired amplification of an audio input signal. However, as is also known, P-channel MOSFETs do not perform as well as N-channel MOSFETs and are more expensive to fabricate and purchase for comparable performance. The inferiority of P-channel MOSFETs is primarily due to inherent differences in N-type and P-type silicon. Consequently, a P-channel MOSFET that is a true electrical complement of an N-channel MOSFET cannot be made using current technology.
Instead of attempting to fabricate a P-channel MOSFET that is a true electrical complement of an N-channel MOSFET, semiconductor manufacturers attempt to match certain parameters of the P-channel and N-channel devices. However, matching of any two parameters of N-channel and P-channel devices causes other parameters of the devices to differ. For example, for the same voltage and current ratings, a P-channel MOSFET will have a higher on-state saturation resistance and a lower transconductance than a similar-sized N-channel MOSFET. To produce a P-channel device that has the same on-state saturation resistance and voltage rating as a particular N-channel device, the silicon die of the P-channel device must be larger than the silicon die of the corresponding N-channel device and, therefore, the P-channel device will cost more than the comparable N-channel device.
Some techniques for producing substantially complementary active devices are also known. For example, until the 1970's, PNP bipolar junction power transistors with performance similar to NPN bipolar junction power transistors were not available. To simulate the function of a PNP power transistor, an NPN power transistor 103 was used with a feedback circuit as depicted in FIG. 1. The feedback circuit consists of a low power PNP transistor 101 and a resistor 105. The combination of the NPN power transistor 103 and the feedback circuit provides an approximately "virtual" PNP transistor circuit 100 and is typically referred to as a "quasi-complementary" circuit. The common application of the virtual PNP transistor circuit 100 was in the output stage of a power amplifier.
Although the virtual PNP transistor circuit 100 provides some complementary operation, it has certain drawbacks. For example, it is difficult to select PNP and NPN transistors 101, 103 for use in the virtual PNP transistor circuit 100 such that the respective direct current (DC) characteristics of the transistors 101, 103 combine to provide a circuit with DC characteristics that are substantially complementary to the DC characteristics of the NPN power transistor 103. Further, variation of the DC characteristics of the NPN power transistor 103, particularly current gain, with varying operating conditions causes the dynamic characteristics of the virtual PNP transistor circuit 100 to deviate from the dynamic characteristics of a true complement of the NPN power transistor 103.
Therefore, a need exists for a circuit that operates in a manner substantially complementary to an amplifying device included therein that can cost-effectively simulate a P-channel MOSFET for use in complementary MOSFET circuit applications and that can maintain complementary operation over varying operating conditions. An apparatus, such as an audio power amplifier or a stereo receiver, that incorporates such a circuit would also be an improvement over the prior art.